Infrared (IR) communication systems in which the transmitted signal may be diffusely scattered are assuming an increasing importance in wireless links among terminals, phones, pagers, PDAs, fax equipment, and other portable devices. IR systems are competitive with radio frequency technologies, which are influenced to a greater extent by interference and spectrum availability. IR is especially attractive in applications where communication takes place over a short range, and for systems which require a high data rate or a low-cost implementation. The Infrared Data Association (IrDA) has created specifications for point-to-point IR systems operating at 4 Mb/s or less, and specifications for other systems are being determined as an ongoing process.
The signal power, P.sub.signal [W], developed in a load resistor, R[.OMEGA.], which results from current generated in a receiving photodiode having responsivity, S.sub..lambda. [A/W], in response to optical signal power P.sub.collected.sbsb.--.sub.signal [W] collected by the receiver optical system is given by EQU P.sub.signal =(P.sub.collected.sbsb.--.sub.signal .multidot.S.sub..lambda.).sup.2 .multidot.R. Eq. 1
The ratio of the mean signal power, S [W], to the noise power, N [W], for the infrared detection process is limited either by thermal noise generated in the photodiode load resistor or its equivalent, by random electron motion, or by shot noise generated in the optical detector itself due to background illumination. When thermal noise current is dominant, the noise power, P.sub.thermal.sbsb.--.sub.noise [W], developed in R is given by EQU P.sub.thermal.sbsb.--.sub.noise =4.multidot.k.multidot.T.multidot.B,Eq. 2
where k is Boltzmann's constant and B [Hz] is the system bandwidth. The S/N for a thermal noise limited system is given by ##EQU1## When shot noise current is dominant, the noise power, P.sub.shot.sbsb.--.sub.noise [W], developed in R is given by EQU P.sub.shot.sbsb.--.sub.noise =2.multidot.e.multidot.S.sub..lambda. .multidot.P.sub.collected.sbsb.--.sub.background .multidot.B.multidot.R,Eq. 4
where e [couloumbs] is the electronic charge and P.sub.collected.sbsb.--.sub.background [W] is the optical power due to background illumination collected by the receiver optical system. The S/N for a shot noise limited system is given by ##EQU2##
For wireless optical systems in a home or office environment, shot noise current due to ambient lighting or sunlight is frequently the dominant source of noise. In addition, such systems commonly employ optical signal sources which incorporate semiconductor light-emitting diodes or diode lasers having an optical output power which is directly proportional to the current through the device. The average dissipation of these semiconductor devices is constrained due to thermal dissipation limitations and health and safety regulations. Moreover, wireless optical systems are frequently a part of portable devices which must use low current to ensure maximum battery life.
Thus, there is a need for an efficient encoding and detection method and device for binary intensity modulated optical data signals.